During the Pacific Exploration of Asian Continental Emission (PEACE) phase A mission in January 2002, we launched ozonesondes in subtropical southeast China at Hong Kong (114.17°E, 22.31°N), middle latitude northeast China at Beijing (116.47°E, 39.81°N), and northwest China at Xining (101.45°E, 36.43°N) in order to study long-range ozone (O3) transport from Eurasia, tropospheric O₃ sources in China, and O₃ outflow to the Pacific. Tropospheric O₃ showed a complex vertical distribution with average tropospheric O₃ columns of 39 ± 4, 23 ± 3, and 30 ± 6 DU in Hong Kong, Beijing, and Xining, respectively, which accounted for 17 ± 2%, 7 ± 1%, and 10 ± 1% of the total O₃ column. The lower tropospheric and boundary layer (BL) O₃ over Xining and especially Beijing exhibited low values, suggesting negligible O₃ formation in middle latitudes of China during the winter season. The results also revealed frequent propagation of enhanced O₃ layers from the lower stratosphere to the upper troposphere over Xining and especially Beijing, suggesting that stratospheric O₃ is an important source of O₃ in the upper troposphere of northern China. This “natural” O₃ is transported downwind by the prevailing westerly wind and acts as a source of O₃ to the east Asian coast and northwestern Pacific. We observed elevated O₃, with a maximum mixing ratio up to 111 ppbv, at 1.5 km in the upper BL over Hong Kong. The elevated O₃ was resulted from transport of pollutants from northwest-central or southwest China and regional O₃ formation and accumulation in south China including the Pearl River Delta and Hong Kong. We also observed enhanced O₃ (>95 ppbv) in the middle and upper troposphere over Hong Kong in air masses transported along the subtropical jet from tropical and subtropical East Africa, south Asia, and Southeast Asia. The O₃ enhancements were most likely due to intrusion of stratospheric O₃ into the troposphere in the Indo-Burmese region of tropical Southeast Asia, where substantial downward motion had been observed.

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